Thanks for all your replies. To answer some of your questions:
They're still living at home because, due to their long drug use and unemployment, they can't afford to move out. The one daughter has a part-time job and a car, but doesn't seem to be saving any money. I've asked her to show me her pay stubs so I can get some understanding of why she can't save enough to move out.
They both see a psychiatrist and take antidepressants.(Their depression preceded their drug use and goes back to their early teens. I too have suffered all my life from depression and believe it to be genetic.) They have no health insurance, though I have a temporary and limited policy on them.
One daughter had surgery in Feb and has just recently started working, also part-time.
I know I have a legal right to throw them out(or I would if hubby would agree), but since they have put a lot of effort into recovery despite relapses, I'd like to make their departure as painless as possible.
As far as treating them unequally, I doubt this will become a problem. I've told them that if the one who moves out goes to recovery house, which I think would be best, I would pay for a year's rent. If she moves anywhere else, I won't pay anything. This would solve the problem of their being together too much and being a bad influence on each other.If they don't agree to the recovery house, I'm sure that one would not move without the other. They are extremely close,which makes their recovery harder(the "people, places, and things" caveat)
I have reminded them that, when they both came home from separate rehabs in December, both rehabs were supposed to place them in recovery houses, but failed to do so. I then agreed to take them both home temporarily.They seem on the surface to accept this, but they are masters of procrastination and rationalization, so my uppermost thought is what deadline to set. One daughter's licens is suspended till November, which limits her employability. I'm thinking of giving them a November or December deadline, but that does seem a bit too lenient.